Earthquakes occur at cracks in the earth's crust in which shifting tectonic plates build stresses within the crust that literally tear the surface of the earth when they are released. Generally speaking, the world's most active fault zones are known. The most famous of these fault zones is the so-called Pacific “ring of fire” circumscribing the Pacific Ocean and including most of the west coast of North, Central and South America, Japan, Taiwan, Indonesia, and New Zealand. Other fault zones occur in, for example, the eastern Mediterranean (e.g., Turkey, Armenia and the Caucasus region of Russia), and in South Asia (for example, Pakistan, Malaysia and Thailand).
Although minor earthquakes are common, with thousands of smaller earthquakes occurring daily, larger magnitude seismic events can cause personal injury, death and property and environmental damage, particularly in heavily populated areas.
Two approaches have been traditionally utilized to prevent or limit damage or injury to objects or payloads due to seismic events. In the first approach, used particularly with structures themselves, the objects or payload articles are made strong enough to withstand the largest anticipated earthquake. However, in addition to the relative unpredictability of damage caused by tremors of high magnitude and long duration and of the directionality of shaking, use of this method alone can be quite expensive and is not necessarily suitable for payloads to be housed within a structure. Particularly for delicate, sensitive or easily damaged payload, this approach alone is not especially useful.
In the second approach, the objects are isolated from the vibration such that the objects fail to experience the full force and acceleration of the seismic shock waves. Various methods have been proposed for accomplishing isolation or energy dissipation of a structure or object from seismic tremors, and these methods may depend in some measure on the nature of the object to be isolated.
Thus, buildings and other structures may be isolated using, for example, passive systems, active systems, or hybrid systems. Such systems may include the use of one or more of a torsional beam device, a lead extrusion device, a flexural beam device, a flexural plate device, and a lead-rubber device; these generally involve the use of specialized connectors that deform and yield during an earthquake. The deformation is focused in specialized devices and damage to other parts of the structure are minimized; however the deformed devices often must be repaired or replaced after the seismic event, and are therefore largely suitable for only one use.
Active control systems require an energy source and computerized control actuators to operate braces or dampers located throughout the structure to be protected. Such active systems are complex, and require service or routine maintenance.
For objects other than buildings, bridges and other structures, isolation platforms or flooring systems may be preferable to such active or deformable devices. Thus, for protection of delicate or sensitive equipment such as manufacturing or processing equipment, laboratory equipment, computer servers and other hardware, optical equipment and the like an isolation system may provide a simpler, effective, and less maintenance-intensive alternative. Isolation systems are designed to decouple the objects to be protected (hereinafter the “payload”) from damage due to the seismic ground motion.
Isolators have a variety of designs. Thus, such systems have generally comprised a combination of some or all of the following features: a sliding plate, a support frame slidably mounted on the plate with low friction elements interposed therebetween, a plurality of springs and/or axial guides disposed horizontally between the support frame and the plate, a floor mounted on the support frame through vertically disposed springs, a number of dampers disposed vertically between the support frame and the floor, and/or a latch means to secure the vertical springs during normal use.
Certain disadvantages to such pre-existing systems include the fact that it is difficult to establish the minimum acceleration at which the latch means is released; it is difficult to reset the latch means after the floor has been released; it may be difficult to restore the floor to its original position after it has moved in the horizontal direction; the dissipative or damping force must be recalibrated to each load; there is a danger of rocking on the vertical springs; and since the transverse rigidity of the vertical springs cannot be ignored with regard to the horizontal springs, the establishment of the horizontal springs and an estimate of their effectiveness, are made difficult.
Ishida et al., U.S. Pat. No. 4,371,143 have proposed a sliding-type isolation floor that comprises length adjustment means for presetting the minimum acceleration required to initiate the isolation effects of the flooring in part by adjusting the length of the springs.
Yamada et al., U.S. Pat. No. 4,917,211 discloses a sliding type seismic isolator comprising a friction device having an upper friction plate and a lower friction plate, the friction plates having a characteristic of Coulomb friction, and horizontally placed springs which reduce a relative displacement and a residual displacement to under a desired value. The upper friction plate comprises a material impregnated with oil, while a lower friction plate comprises a hard chromium or nickel plate.
Stahl (U.S. Pat. No. 4,801,122) discloses a seismic isolator for protecting e.g., art objects, instruments, cases or projecting housing comprising a base plate connected to a floor and a frame. A moving pivoted lever is connected to a spring in the frame and to the base plate. The object is placed on top of the frame. Movement of the foundation and base plate relative to the frame and object causes compression of the lever and extension of the spring, which then exerts a restoring force through a cable anchored to the base plate; initial resistance to inertia is caused due to friction between the base plate and the frame.
Kondo et al., U.S. Pat. No. 4,662,133 describes a floor system for seismic isolation of objects placed thereupon comprising a floor disposed above a foundation, a plurality of support members for supporting the floor in a manner that permits the movement of the floor relative to the foundation in a horizontal direction, and a number of restoring devices comprising springs disposed between the foundation and the floor member. The restoring members comprise two pair of slidable members, each pair of slidable members being movable towards and away from each other wherein each pair of slidable members is disposed at right angles from each other in the horizontal plane.
Stiles et al., U.S. Pat. No. 6,324,795 disclose a seismic isolation system between a floor and a foundation comprising a plurality of ball and socket joints disposed between a floor and a plurality of foundation pads or piers. In this isolation device, the bearing comprises a movable joint attached to a hardened elastomeric material (or a spring); the elastic material is attached on an upper surface of the ball and socket joint and thus sandwiched between the floor and the ball and socket joint; the bearing thus tilts in relation to the floor in response to vertical movement. The floor is therefore able to adjust to buckling pressure due to distortion of the ground beneath the foundation piers. However, the device disclosed is not designed to move horizontally in an acceleration-resisting manner.
Fujimoto, U.S. Pat. No. 5,816,559 discloses a seismic isolation device quite similar to that of Kondo, as well as various other devices including one in which a rolling ball is disposed within the tip of a strut projecting downward from the floor in a manner similar to that of a ball point pen.
Bakker, U.S. Pat. No. 2,014,643, is drawn to a balance block for buildings comprising opposed inner concave surfaces with a bearing ball positioned between the surfaces to support the weight of a building superstructure.
Kemeny, U.S. Pat. No. 5,599,106 discloses ball-in-cone bearings. Kemeny, U.S. Pat. No. 7,784,225 discloses seismic isolation platforms containing rolling ball isolation bearings. Hubbard et al., U.S. Patent Publication 2007/0261323, filed on Mar. 30, 2007 discloses a method and raised access flooring structure for isolation of a payload placed thereupon. Isolation bearings are disclosed in U.S. patent application Ser. No. 13/041,160 filed on Mar. 4, 2011, and Moreno et al., International Patent Application No. PCT/US11/27269, filed on Mar. 4, 2011.
Chen, U.S. Pat. No. 5,791,096 discloses a raised floor system.
Denton, U.S. Pat. No. 3,606,704 discloses an elevated floor structure suitable for missile launching installations with vertically compressible spring units to accommodate vertical displacements of the subfloor.
Naka, U.S. Pat. No. 4,922,670 is drawn to a raised double flooring structure which is resistant to deformation under load. The floor employs columnar leg members that contain a pivot mounting near the floor surface, which permits the floor to disperse a load in response to a side load.
All patents, patent applications and other publications cited in this patent application are hereby individually incorporated by reference in their entirety as part of this disclosure, regardless whether any specific citation is expressly indicated as incorporated by reference or not.